Reversible invert water-in-oil emulsion drilling fluids have been utilized heretofore for drilling well bores penetrating one or more crude oil producing zones. Such emulsion drilling fluids lubricate the drill bit and reduce the overall time required for drilling well bores. These drilling fluids are water-in-oil emulsion, wherein the water or aqueous phase is dispersed within the oil or oleaginous fluid continuous phase. Such water-in-oil emulsions employed as drilling fluids may be stabilized with nonionic amine emulsifiers and have a relatively high pH which insures that the emulsifier retains a non-ionic oil wetting character during drilling.
When a well bore has reached total depth and may have penetrated one or more oil producing zones, the water-in-oil emulsion drilling fluid is then contacted with an externally supplied acid which protonates a nonionic amine emulsifier causing it to take on a cationic charge with an increase in water solubility. By “externally supplied” it is meant that the acid or other fluid is pumped or otherwise introduced into the wellbore from the surface. The resulting protonated water soluble emulsifier has water wetting properties, and as a result, the oil and water phases in the emulsion inverts to produce an oil-in-water emulsion in the well bore. The oil-in-water emulsion may possess a lower viscosity than the water-in-oil emulsion from which it was formed.
The inverted low viscosity oil-in-water emulsion water wets the producing formations which aids in increasing oil production. In addition, the oil-in-water emulsion fluid may be easier to clean up and thus facilitates subsequent operations.
The acid utilized for contacting the water-in-oil emulsion drilling fluid and causing it to invert is generally in the form of an externally supplied aqueous acidic solution which is pumped into the well. However, such operations are known to produce aqueous acid solution-crude oil emulsions and crude oil sludging. Attempts to prevent the formation of aqueous acid solution-crude oil emulsions and crude oil sludging include the use of anionic sulfonate surfactants in the acid solution. While the anionic sulfonate surfactants function well in preventing the formation of aqueous acid-crude oil emulsions and prevent oil sludging from taking place, the anionic sulfonate surfactants may react with the amine emulsifier after it is protonated by the acid, resulting in an emulsifier which does not become water soluble and does not invert the water-in-oil emulsion into an oil-in-water emulsion. This may prevent water wetting and faster clean up, but the aqueous acid solution utilized adds to the internal water phase of the emulsion which results in the emulsion increasing in viscosity. A highly viscous water-in-oil emulsion so-formed may be difficult to remove and may cause damage to the crude oil producing zones penetrated by the well bore. In addition, the intentional addition of acid to invert the well bore represents additional cost and risk in the process.